In recent years, gallium nitride compound semiconductors have become interesting as materials for producing light-emitting devices which emit light of short wavelength. Generally, a gallium nitride compound semiconductor is grown on a substrate made of an oxide crystal such as a sapphire single crystal, a silicon carbide single crystal, or a Group III-V compound single crystal, through a method such as metal-organic chemical vapor deposition (MOCVD), molecular-beam epitaxy (MBE), or hydride vapor phase epitaxy (HVPE).
At present, the crystal growth method that is most widely employed in the industry includes growing a semiconductor crystal on a substrate such as sapphire, SiC, GaN, or AlN, through metal-organic chemical vapor deposition (MOCVD). Specifically, an n-type layer, an active layer, and a p-type layer are grown on the aforementioned substrate placed in a reactor tube, by use of a Group III organometallic compound and a Group V source gas at about 700° C. to about 1,200° C.
After growth of the above layers, a negative electrode is formed on the substrate or the n-type layer, and a positive electrode is formed on the p-type layer, whereby a light-emitting device is fabricated.
Conventionally, such an active layer is formed from InGaN whose composition is controlled so as to modulate the light emission wavelength. The active layer is sandwiched by layers having a bandgap higher than that of InGaN, thereby forming a double-hetero structure, or is incorporated into a multiple quantum well structure on the basis of the quantum well effect.
In a gallium nitride compound semiconductor light-emitting device having active layers included in a multiple quantum well structure, when the thickness of a well layer is adjusted to 20 to 30 Å, satisfactory output is attained, but a problematically high operation voltage is required. In contrast, when the thickness of the well layer is 20 Å or less, operation voltage is lowered, but output is poor.
There has been also proposed a quantum dot structure in which an active layer in the form of a dot pattern is formed as described below.
For example, Japanese Patent Application Laid-Open (kokai) Nos. 10-79501 and 11-354839 disclose light-emitting devices having an active layer of a quantum dot structure. The quantum dot structure is formed through an anti-surfactant effect. However, the above-proposed quantum dot structure has a problem. That is, since the total area of dots (light-emitting dots) is excessively small with respect to the area where current flows, overall emission output with respect to input current is lowered, even though the emission efficiency of each light-emitting dot is enhanced. These patent documents do not stipulate the area covered with dots. However, the area that is not covered with dots is considerably greater than the area covered with dots, as calculated from the dot size and preferred dot density described in the specifications.
In addition, there has been proposed a quantum box structure including a light-emitting box having an area greater than that of a light-emitting dot.
For example, Japanese Patent Application Laid-Open (kokai) No. 2001-68733 discloses an In-containing quantum box structure which is formed by annealing a formed quantum well structure in hydrogen so as to sublimate the well layer. The dimensions of each light-emitting box are preferably as follows: 0.5 nm≦height≦50 nm and 0.5 nm≦width≦200 nm, and a light-emitting box (height: 6 nm, width: 40 nm) is fabricated in a Working Example. Although the light-emitting box density is not stipulated, the area which is not covered with light-emitting boxes is greater than or equal to the area which is covered with light-emitting boxes, as shown in an attached drawing.
Briefly, each of the structures based on the aforementioned techniques do not include quantum dots or quantum boxes in the area on which quantum dots or boxes are not provided. In addition, the area which is covered with quantum boxes or dots is very small and, in contrast, the area which is not covered with quantum boxes or dots is larger.
In such a structure in which the area that is covered with light-emitting boxes or dots is very small and no light-emitting elements are provided in the area that is not covered with quantum boxes or dots, the operation voltage can be lowered, but emission output is problematically reduced. Thus, such a structure cannot be used in practice.
Japanese Patent Application Laid-Open (kokai) No. 2001-68733 also discloses that a quantum box structure is fabricated by forming a conventional quantum well structure and annealing the structure in hydrogen, thereby decomposing an InGaN crystal provided on through-hole dislocations. However, annealing a quantum well structure in hydrogen induces a release of In from a portion to serve as a quantum box structure, thereby blue-shifting the emission wavelength, which is not preferred.
Also, in US Patent Application Publication No. US2003/0160229A1, a quantum well structure, in which a well layer has a thickness which changes periodically, is disclosed. The well layer has depressions and protrusions in upper and lower surfaces, which means that an upper surface of a barrier layer, which fills up the depressions of the well layer, is not flat. In such structure, although the operation voltage can be lowered, emission output is reduced.